The present invention relates generally to very hard compositions of matter and to methods of producing such compositions and relates more particularly to cobalt-free compositions which are very hard and to their methods of preparation.
Various carbides have long been known to exhibit very high hardness values. Tungsten carbide, for example, has a hardness value of 92-94 on the Rockwell A test (i.e., 92-94 R.sub.A). However, pure carbides have also long been known to possess the property of being very brittle. To reduce that brittleness, various materials have been mixed with the carbides as binder materials, which generally act to reduce the hardness but to increase various properties such as the fracture toughness of the compositions.
A binder material which has extensively been used is cobalt, resulting in certain compositions having the very desirable combination of properties of high hardness values (88 to 94.3 R.sub.A) and high fracture toughness values. (See Table 2 in Example II below). Such compositions have found widespread uses, including uses in mining and in machining operations.
However, at present, the U.S. imports about 98% of all cobalt used in this country. Furthermore, its availability has been unreliable and its price has fluctuated wildly in the past several years, ranging from about $6.40 to $50.00 per pound. Therefore, cobalt-free compositions exhibiting high hardness values and high fracture toughness values are extremely desirable now.
Researchers have long attempted to find such a cobalt-free composition. As described by Dr. Paul Schwarzkopf et al. in Cemented Carbides, New York: The MacMillan Company, (1960) at pages 188-190, recently there has been a successful replacement of cobalt by 3:1 Fe-Ni alloys in tungsten carbide compositions; and Schwarzkopf et al. estimated that Co as binder material can be replaced by Fe-Ni in about 90-95% of all carbides. Additionally, at pages 214-215, was the statement that in addition to the carbides of the transition metals of groups IV-VI, a number of nitrides, borides, and silicides of these metals and various intermetallic compounds and nonmetallic substances such as oxides and other ceramics, silicon carbide, and boron carbide should be considered as basis for potential tool materials. The reference added, however, that most of these substances cannot be bonded to form solids of satisfactory strength and toughness, and only aluminum oxide and boride materials can compete with cemented carbides. The reference does not teach one that a very hard composition can be produced by using only a minor amount of carbide, and in particular it does not teach using the type of carbide in an amount within the narrow range, as described below.
In U.S. Pat. No. 3,386,812, 80 v/o Ni and 20 v/o B.sub.4 C are mixed and then cast to form a composition which is 93 w/o Ni and 7 w/o B.sub.4 C and which has a hardness of 1100 DPH. However, a considerably harder material was sought.
Despite major R and D efforts to find substitutes for the hardest available cobalt-bonded materials, a need still exists for a very hard cobalt-free composition which requires only a minor amount of a particular carbide.